Krabbe's disease, or globoid cell leukodystrophy, is caused by mutations in the gene coding for the lysosomal enzyme, galactocerebrosidase (GALC), which is required to digest galactocerebroside in myelin forming oligodendrocytes and Schwann cells. A pathological hallmark of this rapidly progressive demyelinating disease is the marked accumulation of macrophages (globoid cells) in the nervous system. However, the function of these cells is unclear. To date, transplantation of bone marrow stem cells and umbilical cord blood cells are the only treatments that have proven beneficial, with donor-derived microglia potentially supplying enzyme. However, the majorities of patients with infantile onset are untreatable and succumb by two years of age. Our long term goal is to devise an efficient therapy for patients at any stage of the disease by enhancing enzyme replacement and by extending the time window for successful therapy. The twitcher mouse (twi), which has a mutation in the GALC gene, is considered a bonafide model of Krabbe's disease. Recently we created macrophage-deficient twi mice (twi+op) by crossing twi and macrophage-deficient osteopetrotic mice (op), the only model of Krabbe's disease from which macrophages are eliminated. Our preliminary study of twi+op indicates that the absence of microglia/macrophages exacerbates the demyelinating pathology of twi. We have also shown that twi oligodendrocytes can be maintained for long periods of time when they have access to enzyme in vivo. The specific hypotheses guiding this proposal are that microglia/macrophages are beneficial in the demyelinating pathology of twi and these cells can be used as a therapeutic vehicle to supply GALC to mutant myelinating cells. To test these hypotheses, we propose three specific aims. 1) Determine the roles of microglia/macrophages in the demyelinating pathology of twi. Using a multidisciplinary approach, the exacerbated demyelination in twi+op will be analyzed and the underlying molecular mechanisms will be explored. 2) Determine whether microglia and macrophages transplanted locally or systemically as a means of enzyme replacement can alleviate the demyelination of twi. 3) Determine whether transplantation of GALC-overexpressing bone marrow stem cells and umbilical cord blood cells infected with a GALC-gene-encoding lentivirus can outperform the conventional bone marrow transplantation in twi. These combined data will provide the foundation required to devise therapeutic approaches to treating patients at all stages of the disease.